Following is a summary of the design principles governing XLink:

XLink must be straightforwardly usable over the Internet.

Three standards have been especially influential:

HTML: Defines several SGML element types that represent links.

Many other linking systems have also informed this design, especially Dexter, FRESS, MicroCosm, and InterMedia.

The following basic terms apply in this document. arc

A symbolic representation of traversal behavior in links, especially the direction, context and timing of traversal.

The formal grammar for locators is given using a simple Extended Backus-Naur Form (EBNF) location, as described in the XML specification.

The only locator attribute at this time is href. This attribute must contain either a string in the form of a URI that defines the remote resource being linked to, a string containing a fragment identifier that links to a local resource, or a string containing a URI with a fragment identifier concacenated onto it.

Arcs contain two attributes, from and to. The from attribute may contain a string containing the content of a role attribute from the resource being linked from. The purpose of the from attribute is to define where this link is being actuated from.

The to attribute may contain a string containing the content of a role attribute from the resource being linked to. The purpose of the to attribute is to define where this link traverses to.

The application may use this information in a number of ways, especially in a complex hypertext system, but it is mainly useful in providing context for application behavior.

There are two attributes associated with behavior: show and actuate. The show attribute defines how the remote resource is to be revealed to the user. It has three options: new, parsed, and replace. The new option indicates that the remote resource should be shown in a new window (or other device context) without replacing the previous content. The parsed option, relating directly to the XML concept of a parsed entity, indicates that the content should be integrated into the document from which the link was actuated. The replace option is the one most commonly seen on the World Wide Web, where the document being linked from is entirely replaced by the object being linked to.

The actuate attribute defines how the link is initiated. It has two options: user and auto. The user option indicates that the link must be initiated by some sort of human-initiated selection, such as clicking on an HTML anchor. The auto option indicates that the link is automatically initiated when the application deems that the user has reached the link. It then follows the behavior set out in the show option.

There are two attributes associated with semantics, role and title. The role attribute is a generic string used to describe the function of the link's content. For example, a poem might have a link with a role="stanza". The role is also used as an identifier for the from and to attributes of arcs.

The title attribute is designed to provide human-readable text describing the link. It is very useful for those who have text-based applications, whether that be due to a constricted device that cannot display the link's content, or if it's being read by an application to a visually-impaired user, or if it's being used to create a table of links. The title attribute contains a simple, descriptive string.

Simple links can be used for purposes that approximate the functionality of a basic HTML A link, but they can also support a limited amount of additional functionality. Simple links have only one locator and thus, for convenience, combine the functions of a linking element and a locator into a single element. As a result of this combination, the simple linking element offers both a locator attribute and all the behavior and semantic attributes.

The following are two examples of linking elements, each showing all the possible attributes that can be associated with a simple link. Here is the explicit XLink simple linking element. <!ELEMENT xlink:simple ANY> <!ATTLIST xlink:slink href CDATA #REQUIRED role CDATA #IMPLIED title CDATA #IMPLIED show (new|parsed|replace) "replace" actuate (user|auto) "user" > And here is how to make an arbitrary element into a simple link. <!ELEMENT xlink:simple ANY> <!ATTLIST foo xlink:type (simple|extended|locator|arc) #FIXED "simple" xlink:href CDATA #REQUIRED xlink:role CDATA #IMPLIED xlink:title CDATA #IMPLIED xlink:show (new|parsed|replace) "replace" xlink:actuate (user|auto) "user" > Here is how the first example might look in a document: <xlink:simple href="http://www.w3.org/TR/wd-xlink" role="working draft" title="The XLink Working Draft" show="replace" actuate="user"> The XLink Working Draft.</xlink:simple> <foo xlink:href="http://www.w3.org/TR/wd-xlink" xlink:role="working draft" xlink:title="The XLink Working Draft" xlink:show="new" xlink:actuate="user"> The XLink Working Draft.</foo> Alternately, a simple link could be as terse as this: <foo xlink:href="#stanza1">The First Stanza.</foo>

There are no constraints on the contents of a simple linking element. In the sample declaration above, it is given a content model of ANY to illustrate that any content model or declared content is acceptable. In a valid document, every element that is significant to XLink must still conform to the constraints expressed in its governing DTD.

Note that it is meaningful to have an out-of-line simple link, although such links are uncommon. They are called "one-ended" and are typically used to associate discrete semantic properties with locations. The properties might be expressed by attributes on the link, the link's element type name, or in some other way, and are not considered full-fledged resources of the link. Most out-of-line links are extended links, as these have a far wider range of uses.

An extended link differs from a simple link in that it can connect any number of resources, not just one local resource (optionally) and one remote resource, and in that extended links are more often out-of-line than simple links.

These additional capabilities of extended links are required for:

Enabling outgoing links in documents that cannot be modified to add an inline link

Application software might be expected to provide traversal among all of a link's participating resources (subject to semantic constraints outside the scope of this specification) and to signal the fact that a given resource or sub-resource participates in one or more links when it is displayed (even though there is no markup at exactly that point to signal it).

A linking element for an extended link contains a series of child elements that serve as locators and arcs. Because an extended link can have more than one remote resource, it separates out linking itself from the mechanisms used to locate each resource (whereas a simple link combines the two).

The xlink:type attribute value for an extended link must be extended, if the link is being instantiated on an arbitrary element. Note that extended links introduce variants of the show and actuate behavior attributes. These attributes, the showdefault and actuatedefault define the same behavior as their counterparts. However, in this case, they are considered to define the default behavior for all the linking elements that they contain.

However, when a linking element within an extended link has a show or actuate attribute of its own, that attribute overrides the defaults set on the extended linking element.

The extended linking element itself retains those attributes relevant to the link as a whole, and to its local resource if any. Following are two sample declaration for an extended link. The first is an example of the explicit XLink extended link: <!ELEMENT xlink:extended ((xlink:arc | xlink:locator)*)> <!ATTLIST xlink:extended role CDATA #IMPLIED title CDATA #IMPLIED showdefault (new|parsed|replace) #IMPLIED actuatedefault (user|auto) #IMPLIED > The second is an example of an arbitrary element being used an extended link: <!ELEMENT foo ((xlink:arc | xlink:locator)*)> <!ATTLIST foo xlink:type (simple|extended|locator|arc) #FIXED "extended" xlink:role CDATA #IMPLIED xlink:title CDATA #IMPLIED xlink:showdefault (new|parsed|replace) #IMPLIED xlink:actuatedefault (user|auto) #IMPLIED > The following two examples demonstrate how each of the above might appear within a document instance. Note that the content of these examples would be other elements. For brevity's sake, they've been left blank. The first example shows how the link might appear, using an explicit XLink extended link: <xlink:extended role="address book" title="Ben's Address Book" showdefault="replace" actuatedefault="user"> ... </xlink:extended> And the second shows how the link might appear, using an arbitrary element: <foo xlink:type="extended" xlink:role="address book" xlink:title="Ben's Address Book" xlink:showdefault="replace" xlink:actuatedefault="user"> ... </foo>

An arc is contained within an extended link for the purpose of defining traversal behavior. More than one arc may be associated with a link. Otherwise, arc elements function exactly as the arc attributes might lead on to expect.